Despite the recent advancements in smoking cessation intervention treatments, the majority of smokers relapse soon after a quit attempt. Compared to men, women are at an increased risk for relapse. Although the clinical evidence is lacking, recent research suggests that allopregnanolone (ALLO) may be protective against negative drug abuse behaviors such as escalation of self-administration and severity of withdrawal symptoms. ALLO is a stress-reducing neuroactive steroid metabolized primarily from the sex hormone progesterone; therefore, in women, it varies by menstrual phase (low levels in follicular phase; high levels in luteal phase). Employing a multidisciplinary team with expertise i tobacco research, neuroendocrine response, sex hormones and analytical methodology, we will address the following project objective - investigate the role of ALLO as a possible underlying neurobiological protective factor against smoking relapse. We will achieve this objective by conducting a secondary data analyses from an ongoing study that utilizes a 2x2 randomized cross-over design to assess the role of menstrual phase and mood on short-term smoking abstinence. Our analysis will include assessment of menstrually-timed serum samples, self-reported measures of withdrawal, and response to a nicotine challenge during short-term smoking abstinence. This adjunct study provides a unique opportunity to assess our specific study aims: (1) to confirm the menstrual phase differences in systemic levels of ALLO during short-term smoking abstinence, (2) to identify the menstrual phase differences in systemic levels of ALLO in response to a nicotine challenge during short-term smoking abstinence, and (3) to determine the effect of ALLO on withdrawal symptoms and response to a nicotine challenge during short- term smoking abstinence. If funded, this project will result in new information that will shed light on the role a sex hormones metabolite, and its possible contribution in the high rates of smoking relapse among women. This information will directly inform new areas of research on smoking cessation and relapse prevention interventions. PUBLIC HEALTH RELEVANCE: This project aims to characterize the role of allopregnanolone in short-term smoking abstinence in female smokers. Allopregnanolone, a neuroactive steroid that is known to reduce stress, is metabolized from the sex hormone progesterone and may be protective against negative drug abuse behaviors such as escalation of drug use. If allopregnanolone is protective against smoking relapse, this information could be incorporated in smoking cessation interventions to improve smoking cessation rates in women.